[doodle]

I have been making butter tarts for over 40 years, but only mixing two dozen at a time for our own enjoyment. Never have I not had my sugar dissolve - ie. go "grainy" on me after they have been cooked and cooled.
Now I have to tell you, my SIL is making these for us at our brand new gluten-free baking shop so I have not seen every thing he has done that may have caused this to happen.
Here is what I do know.
The shells, which we make ourselves, are much shallower than my home muffin tins I have always used.= Wider, shallower baking surface.
I think perhaps he was whipping the egg into the mixture too long which I think caused the excess foam on the top so I asked him to mix, not beat in the egg next time. ?right?
The sugar not only was "grainy" but formed a thin solid bottom on the tart shell. We are using our own gluten-free pastry which we mix and roll/press ourselves into the tart shells, but I can't see how the pastry componnets would matter? We "press" our pastry balls into the shell with a great gadget we have but we don't make them rock solid.
The brown sugar was purchased from a bakery supplier and on closer examination, it appears courser/grainier than the regular grocery store sugar I have always used. I didn't realize there would be much of a difference and thought it would all "melt" anyway we are not talking a big difference in the size of the grains but it is noticeable once you take the time to compare.
The ingredients are as follows:
butter, brown sugar, egg, vanilla, (a bit of -vinegar to tone down the sweetness, cream to add to the richness & corn syrup to maintain the viscosity) and of course, raisins which have been washed and slightly pumped with the water they soak in but patted dry.
What the heck do you think is going wrong? Oh, BTW, I have a great deal of this brown sugar. If you think that may be the probem, do you have any suggestions as to what do do - other than trying to "grind" it finer myself?
thanks so much for any advice, Doodle

[Dillbert]

one possibility is the sugar is not getting dissolved in an aqueous liquid - it's in a supersaturated solution of (?)egg(?) and then falling out of solution.

what is the assembly of ingredients sequence/routine?

[doodle]

thanks Dillbert - Great question & I had wondered if the order would make a difference. I do know he has not had a great deal of experience in the world of baking pastries from scratch but has mastered many of the other gluten-free items for our purposes.
but-
I do not know his blending method. I haven't seen him prepare the filling but I will ask him and get back to you on that.
thanks, doodle

[ChrisLehrer]

You might try blenderizing the sugar first to make it like caster sugar. But I don't know how well that would work with brown sugar. There is such a thing as granulated brown sugar -- that could probably be blenderized very fine.

[Gunnar]

the only thing i can think of is to really blend that butter and sugar together first over a double boiler. once it's well incorporated then i would add egg (probably let the butter and sugar cool a bit) and blend. As with all baking the chemistry can be changed by the smallest things, best of luck.